The overall aim of the proposed studies is to determine the physiological and morphological characteristics of vestibular neurons pathways which are involved in controlling eye movement in the squirrel monkey; particularly in the horizontal plane. Ther specific aims of the proposal are to: 1) determine the morphology, axonal projections and physiological characteristics of vestibular neurons which are directly involved in the horizontal vestibuloocular reflex (VOR), or which project directly to the abducens nucleus or the medial rectus subdivision of the oculomotor nucleus. 2) To determine the location, dendritic morphology, axonal projections and physiological characteristics of type II vestibular neurons, to delineate the morphological classes of type II neurons, and to study the physiological activity of each class in respect to vestibular and visual stimuli and to eye movements. 3) To study the morphology and physiological activity of other classes of vestibular neurons whose activity is modulated during horizontal head acceleration and/or horizontal eye movements. The methods to be used in the proposed studies include anatomical tracer methods, and intracellular recordings and horseradish peroxidase injections into physiologically identified vestibular neurons in anesthetized and alert squirrel monkeys. The results of these studies are expected to increase our knowledge of the anatomical and physiological characteristic of central vestibular neurons and pathways, and in general our understanding of the brainstem control of vestibular and oculomotor functions.